The present invention relates to a magnet device, for instance, a magnet device used for magnetic resonance imaging apparatus because of its formation of a static magnetic field space between main coils in a pair positioned facing each other.
Magnetic resonance imaging apparatus uses nuclear magnetic resonance phenomena taking place when a test object (examinee) placed in a static magnetic field space is emitted a high-frequency pulse and thus can obtain images representing the physical and chemical properties of the test object. Accordingly, the magnetic resonance imaging apparatus are now in use particularly for medical purposes. The magnetic resonance imaging apparatus (hereinafter MRI apparatus) are in general made up chiefly of a magnet device for working as a magnetic field generation source for applying a static magnetic field within the imaging region where the test object is carried to, an RF coil for emitting high-frequency pulses toward the imaging region, a receiving coil for receiving responses from the imaging region and a gradient magnetic field coil for applying a gradient magnetic field for giving positional information on resonance phenomena in the imaging region.
To enhance the image quality of an MRI apparatus, it is demanded to increase the homogeneity of magnetic field in the imaging space. For instance, in the magnet device of the MRI apparatus, the entrance to the imaging region of external magnetic flux caused by another magnetic field generation source impairs the homogeneity of the magnetic field. This necessitates the warding-off of the external magnetic flux.
To ward off such external magnetic flux, a magnet device of the style in which a cylindrical magnet has horizontally directed magnetic fields generating in it has been treated with various solutions. For instance, in EP0299325B1 describes an external magnetic flux warding-off method in which a bridge cable is used to short-circuit the coil generating a magnetic field in the same direction as the static magnetic field and the coil generating a magnetic field in the opposite direction as the static magnetic field. JP3447090B describes an external magnetic flux warding-off performance optimizing method in which a bridge cable is used to tap in the middle of the coil winding for short-circuiting. But it is not easy to manufacture a magnet so constructed that a coil series, particularly, windings are tapped using a bridge cable.
On the other hand, U.S. Pat. No. 4,931,735 and JP2001-196219A describe an external magnetic flux warding-off method in which a main coil for working as a magnetic field generation source is provided with a shielding coil to ward off external magnetic flux. U.S. Pat. No. 4,931,735 describes a method in which a magnet device has a solenoid-shaped shielding coil built into it. JP2001-196219A describes a construction in which a cylindrical shielding coil is placed coaxially to a main coil, sharing a persistent current switch control power source with the main coil. These methods pose a problem in that a magnet device tends to have undesirably long axial length. For this reason, U.S. Pat. No. 5,329,266 describes a method in which a main coil have a shielding coil with desirably few turns of winding placed in its vicinity and connected in series so that both of them can generate magnetic fields in the same direction. JP3043494B describes a method in which a plurality of shielding coil different in radius are built in.